Resonant free-piston Stirling engines are known to the industry and have been described in a number of prior art patents, periodicals and textbooks, such as the textbook entitled "Stirling Engines" by G. Walker, published by Clarendon Press--Oxford, England--1980.
In free-piston Stirling engines (which may or may not operate resonantly) there are mechanical the thermodynamic requirements which work at cross purposes. This is particularly true if the displacer is partially powered by some means external to the Stirling engine cycle, such as the free-piston Stirling engine design described in U.S. Pat. No. 4,215,548-issued Aug. 5, 1980, for a "Free-Piston Regenerative Hot Gas Hydraulic Engine," for example.
In order to reciprocally support the displacer within the Stirling engine housing, a rod is required which must be large enough to provide mechanical stiffness sufficient to prevent the displacer forces from flexing the rod excessively, and which employs reasonable size bearings for the displacer and rod assembly.
Opposing the above-noted requirement, is the need for an unbalanced area on the displacer/rod drive assembly which is acted upon by the engine pressure wave, and the further requirement that the unbalanced area not be too large or the displacer will overstroke, making free oscillation difficult to achieve.
A further complication arises in the known free-piston Stirling engine designs due to the practice of using the displacer rod diameter to establish the size of the piston of the displacer gas spring which in combination with the displacer mass forms a spring-mass system with a natural frequency, substantially the same as the engine operating frequency. In such designs, the small rod area complicates the displacer gas spring design since extremely high pressure ratios (and high losses) must be accepted to achieve required stiffness for the displacer spring.
In order to overcome the above-noted difficulties in known free-piston Stirling engine designs, the present invention was devised.